The goals of this proposal are to investigate the role of proto-oncogenes in normal mammalian development and to understand how aberrations in the expression of these genes could result in pregnancy wastage or congenital malformations. C-fos and c-jun proto-oncogenes function together to regulate gene expression at the molecular level and have been chosen for this study as proto-oncogenes implicated in development. C-fos is expressed in a tissue-specific manner during embryogenesis and shows high levels of expression in extraembryonic tissues. These tissues play an essential role in implantation and in the survival of the embryo during the postimplantation period. It is our hypothesis that tissue-specific expression reflects a functional requirement for the c-fos gene product. We further postulate that c-jun will be expressed in a similar manner and will also be required during development. The specific aims of this proposal are to clarify the extent of c-fos and c-jun expression throughout the pre- and early postimplantation period in the mouse and examine the functional role of these genes in development. This will be accomplished by the following specific aims: Specific Aim I. We will detail the expression of c-fos and c-jun during pre- and postimplantation mouse development using in situ hybridization for RNA localization and immunohistochemistry for localization of the proteins. Specific Aim 2. We will produce embryonic stem (ES) cell lines heterozygous for mutated c-fos and c-jun by gene targeting via homologous recombination. Preliminary results have been successful in targeting c-fos and one mutated clone is now available. Specific Aim 3. We will then produce transgenic animals heterozygous and homozygous for mutated C-fos or c-jun by way of ES cell chimeras, in order to determine the effect of a lower dose or absence of c-fos or c-jun gene product on development. One series of chimeras is already being test-bred to produce heterozygous mice. If we determine that heterozygous cells are not capable of completing gametogenesis in a chimera, an alternative, rescue protocol will be used that will allow the study of gametogenesis in heterozygous males.